1. Field of the Invention
The invention is directed to an embossing assembly and methods for its preparation.
2. Description of Related Art
U.S. Pat. No. 4,923,572 (hereinafter referred to as the '572 patent) discloses a generally cylindrical image embossing tool that can be used for embossing a material on a web. The method for the manufacture of the image embossing tool involves multiple steps, including (1) placing an embossable material around the surface of a rigid cylinder, followed by coating a thin metal, such as silver, over it, (2) stamping a desired image or pattern onto the embossable layer with a stamper, (3) electroforming to form a nickel electroform on the outer surface of the embossable layer, (4) applying a reinforcement layer over the electroform, (5) removing the rigid cylinder; (6) stripping the embossable layer to form a plating mandrel, (7) forming a second electroform on the interior of the plating mandrel and (8) separating the plating mandrel from the second electroform. According to the '572 patent, multiple copies of the second electroform can be prepared in the same manner and then be placed over a carrier cylinder or a plurality of rollers to form an embossing tool to allow continuous embossing. This embossing tool and its manufacturing process, however, suffer several disadvantages. For example, the process requires the stamping surface of the stamper to have a curvature same as that of the embossable material on the rigid cylinder. This is difficult to accomplish in practice. Secondly, if there are defects on the stamper, the defects will be carried over to copies of the electroforms prepared from the same stamper. Thirdly, it is also difficult to achieve defect-free joint lines between two adjacent stamps.
U.S. Pat. No. 5,327,825 (hereinafter referred to as the '825 patent) discloses a method for making a die through embossing or microembossing. More specifically, the method involves embossing a pattern or design onto a silver layer coated on a cylindrical surface, via the use of a concave-shaped stamping surface which carries the pattern or design to be imparted onto the silver layer and has a radius matching the radius of the cylindrical surface. This microembossing step is carried out multiple times so that the die prepared from the method has a repeated pattern or design from the concave-shaped stamping surface. This method has disadvantages similar to those of the process of the '572 patent, e.g., difficulty in matching the curvature of the stamping surface and the cylindrical surface; repeated defects resulted from an imperfect stamping surface; and difficulty in achieving defect-free joint lines between adjacent stamps.
U.S. Pat. No. 5,156,863 (hereinafter referred to as the '863 patent) discloses a method for manufacturing a continuous embossing belt. The method involves combining a series of “masters” or “copies” in a cluster to provide a desired pattern in a fixture and an electroform strip made of the cluster. The embossing belt is formed after multiple electroforming steps starting from a master cluster fixture. One of the drawbacks of this method is the difficulty to generate individual masters or copies for the cluster with same thickness. Therefore, there will be height differences between adjacent masters or strips that will result in formation of defect lines on the final embossed product. In addition, it is also difficult to avoid damage on the sleeve-type mandrel and the shim during their separation, particularly when a complicated microstructure with a deep 3D profile is involved.
U.S. Pat. Nos. 5,881,444 and 6,006,415 disclose a method for forming print rolls bearing holograms. The hologram pattern is formed by laser etching on the surface of a photoresist coated on a piece of flat glass or metal substrate. Mother shim and subsequent sister shims are electroformed as a flat plate. Then, a sister shim is mounted on the print roll to obtain an embossing tool. The disadvantages of the method include formation of defective joint lines resulted from rolling and welding a flat shim to a cylinder, and the difficulty in the adjustment of concentricity of the sister shim and the print roll. If the shim and roll are not concentric, the embossing pressure will not be uniform which will produce embossed microstructures with poor fidelity.